What Is a Quartz Crystal?
A quartz crystal unit is a passive element that vibrates at a specific frequency and is sometimes referred to as a piezoelectric or piezoelectric element. Initially, natural quartz crystals were used, but as demand increased, they were replaced by synthetic quartz crystals, such as silicon dioxide, barium titanate, and Rochelle salt.
Piezoelectric elements, in combination with oscillation circuits, are built into household appliances such as cellular phones, televisions, and digital cameras, as well as electronic devices such as automobiles and medical equipment. Quartz crystal units’ thickness and cutting method vary depending on the frequency range used. For example, the cutting method known as AT cut can be used to obtain quartz crystal units with frequencies ranging from 1 to 300 megahertz and usable in a wide temperature range.
Principle of Quartz Crystals
The piezoelectric crystal is a material in which a surface charge proportional to pressure appears when pressure is applied in a specific direction. The phenomenon in which a surface charge is generated in proportion to pressure is called the piezoelectric effect or piezoelectric phenomenon. Conversely, the phenomenon in which the crystal is deformed by the application of voltage is called the reverse piezoelectric effect. Quartz crystals were literally made from natural quartz crystals in the beginning, but nowadays, synthetic quartz crystals are used to meet increasing demand. Silicon dioxide, potassium titanate, Rochelle salt, and other materials are used instead.
Quartz crystals have electrodes attached to them, and an electric current flows through them from an external source. The surface charge of the quartz crystal changes in accordance with the phase of the current, causing periodic deformation of the quartz crystal. Quartz crystal units utilize the vibration phenomenon caused by this periodic shape change.
Applications of Quartz Crystals
Quartz crystal units are used in a variety of household electronic devices such as cell phones, televisions, digital cameras, and personal computers. In addition, they can also be found in automobiles and medical equipment because they are stable with slight frequency fluctuation. Oscillation circuits based on quartz crystal units are much more accurate than other oscillation circuits on the ppm order.
Generally used quartz crystal units are cut very thin, with thicknesses ranging from 20 to 50 micrometers. In addition, electrodes are attached to the plate surface of the quartz crystal unit for connection to external terminals and electrodes. The thickness of the quartz crystal and the surface from which it is cut vary depending on the functions required of the quartz crystal.
Frequency of Quartz Crystals
The frequency of a quartz crystal unit varies depending on the way the crystal is cut and its thickness. The frequency also varies depending on the cutting method. For example, the “AT cut” method can produce quartz crystals that can be used in a wide temperature range, corresponding to frequencies from 1 to 300 megahertz.
The “BT cut” method, which has a different cutting angle, can be used in the frequency range of 7~38 megahertz, and the amount of frequency change in response to temperature is different compared to the AT cut method. In addition, quartz crystals cut into a tuning fork shape can handle 32.768 kilohertz and are used in clocks.
Load Capacitance
When incorporating a crystal unit into a circuit, it is necessary to match the load capacitance of the circuit and the crystal unit. Load capacitance is the value of equivalent capacitance in virtual series when looking at the oscillation circuit from the crystal unit side. Since the frequency variation in response to changes in load capacitance varies depending on the value of load capacitance, it is necessary to incorporate a crystal unit with appropriate load capacitance to stabilize the circuit.
When a crystal unit is actually used, the circuit is matched based on the crystal unit’s oscillation frequency, tolerance deviation, and load capacitance value. However, in an actual circuit, there are stray capacitances generated by various factors, which may cause a deviation from the nominal load capacitance of the crystal unit. Therefore, after determining the difference between the oscillation frequency when the crystal unit is built into the circuit and the standard load capacitance oscillation frequency, adjustment is made by fine-tuning the capacitance of the circuit to bring the difference close to zero.